Multiband slice accelerated imaging has recently been demonstrated in gradient recalled echo (GRE) and echo planner imaging (EPI) sequences to improve imaging efficiency. In a multiband acquisition, two or more slices are excited simultaneously using a multiband RF pulse and the signals from all slices are acquired simultaneously by multiple receive coils. These aliased signals can be unwrapped by using the receive coil sensitivity profiles to generate separated images of the independent slices. This method offers acceleration in imaging speed equal to the number of slices that are excited simultaneously.
One of the key components of a multiband acquisition is to measure the spatial sensitivity profiles of the receive coils prior to multiband image reconstruction, typically with the acquisition of an unaccelerated, or “single-band,” reference scan. These sensitivity profiles are used during parallel imaging reconstruction to separate the aliased slices. In conventional multiband imaging methods, the single band reference scan is acquired using identical imaging sequence as the slice accelerated scan. However, such a reference scan acquisition is not efficient with sequences such as spin echo (SE), turbo spin echo (TSE), and GRE with long TRs, sequences with high spatial resolution, and sequences with magnetization preparation. Furthermore, if there are no repeated measurements required, a full single-band reference scan obviates the need for any following slice accelerated imaging scan.
Another key component of robust multiband acquisition is to minimize noise amplification and residual aliasing artifact after parallel imaging reconstruction. By introducing desired field-of-view (FOV) shift through modulating the phase of the multiband RF excitation pulse among simultaneously acquired slices, the Controlled Aliasing In Parallel Imaging Results In Higher Acceleration (CAIPIRINHA) technique effectively increases the distance between aliased pixels and improves slice separation with reduced G-factor penalty and residual aliasing artifact. While this RF modulation CAIPIRINHA approach can be used for conventional SE, GRE, and steady-state free precession (SSFP) acquisitions, where one phase-encoding (PE) line of k-space is acquired following each excitation, it is not applicable to sequences that acquire multiple k-space lines after one excitation, such as EPI and TSE. Following the idea of line by line phase modulation, slice-selective gradients blips were introduced to generate phase modulation on k-space lines of an EPI sequence for FOV shifting. With proper cycling sets of slice-selective gradients blips across k-space lines, the blipped CAIPIRINHA method allows desired FOV shift with substantially mitigated phase accruals along the phase-encoding direction, thus minimizing voxel titling and slice blurriness for an EPI sequence. However, the conventional blipped CAIPIRINHA method is not suitable for sequences using refocusing RF pulses to generate additional spin echoes such as, for example, TSE, half-fourier acquisition single-shot turbo spin-echo (HASTE), and gradient and spin echo (GRASE). Because the blipped CAIPIRINHA method applies only one gradient blip to achieve desired accumulated gradient moment for phase modulation of each k-space line, it will keep various phase accruals across refocusing RF pulses to the following readouts, which violates the critical Carr Purcell Meiboom Gill (CPMG) conditions. In addition, the blipped CAIPIRINHA method is not compatible with the balanced SSFP sequence either, because the gradient blip induced phase is not balanced before the subsequent RF excitation.